Broadcast allows the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every user on the network. Upon receiving a packet and if a user is not intended to receive the message, it discards the received packets. In contrast to this the point-to-point communication means the sending of a message from a single sender to a single receiver. Some broadcast systems also supports transmission to a subset of the users, the so-called multicasting. Multicasting is a service that permits sources to send a single copy of the same data to an address that causes the data to be delivered to multiple recipients, who are registered to a multicast group. Therefore multicasting is a form of broadcasting. The difference is that in case of broadcasting the user is not requested to register to any group, the message is distributed to all users.
In broadcasting and also in multicasting only one copy of a message passes over any link in a network and copies of the message are made only where paths diverge. From the network perspective, broadcast dramatically reduces overall bandwidth consumption, since the data is replicated in the network at appropriate points rather than in the end-systems. Further a server, which is sending broadcast messages, needs to manage only one session.
In the following the registration procedure of Internet Protocol multicast will be handled in more details.
In case the multicast is used in Internet Protocol IP network then it is called IP multicast. Among other features characterising the IP multicast, a specific form of addressing is applied using the existing IP address, like the so-called D-address in IP version 4.
Within the IP multicast the membership of a multicast session group is dynamic which means that hosts may join and leave groups at any time. In order to allow hosts on networks to indicate whether they wish to join or leave a particular multicast group there is a protocol called the Internet Group Message Protocol IGMP. Thus this protocol lets the system know which hosts currently belong to which multicast group. This information is required by the multicast routers to know which multicast data packet is to be forwarded onto which interface.
The IGMP is a part of the IP layer and the IGMP messages are transmitted in IP data packets. The version 1 of IGMP is described in RFC 1112 “Host extensions for IP multicasting” S. E. Deering, Aug. 1, 1989, RFC 2236 “Internet Group Management Protocol, Version 2” W. Fenner, November 1997 describes the version 2. The IGMP has been developed for IP version 4. In Internet Protocol IP version 6 there is a similar protocol called Multicast Listener Discovery MLD, which is used for the same purpose as the IGMP. MLD provides more functionality as IGMP. The description of the first version of MLD can be found in RFC 2710 “Multicast Listener Discovery (MLD) for IPv6” S. Deering, W. Fenner, B. Haberman, October 1999. However the messages used in MLD correspond to the IGMP messages. In the following the IGMP will be used as an example. Although this should not be restricted to the IGMP, the functionality of the invention is also given by usage of MLD.
In principle the IGMP uses two basic messages to fulfil its tasks, the membership report and the membership query message and the following rules are applied.
A multicast router sends a membership query at regular intervals to see if any hosts still belong to any group. The router must send one query out each interface. A host responds to a membership query by sending one membership report for each group that still contains at least one process. A host joins a group sending also the membership report.
Using the information received by applying the report and the query messages a table with its interfaces having at least one host in a multicast group is established. After reception of the multicast data, the router forwards the data out on those interfaces, which have at least one member.
With IP multicast receivers do not need to know who or where the senders are to receive traffic from them and the senders never need to know who the receivers are Neither senders nor receivers need to care about the network topology as the network optimises delivery. The distribution of the information via the IP multicast is performed on the base of a hierarchical connection of the hosts, like for example a multicast delivery tree. Several algorithms have been proposed for building multicast distribution trees, like for example spanning trees, shared-trees, source-based trees, and core-based trees. The descriptions of the corresponding algorithms can be found in “IP telephony: Packet-based multimedia communications systems” O. Hersent, D. Gurle, D. Petit, Addison-Wesley, Wesley, Harlow, 2000. After the establishment of the multicast delivery tree, the distribution of the information is done by the IP multicast routing protocols. The detailed description of the corresponding IP multicast routing protocols can be also found in the above-mentioned document.
In order to explain the problem occurring by introduction of broadcast in a point-to-point oriented packet-switched telecommunication system in the following an overview of the architecture of the General Packet Radio System GPRS network is given.
The GPRS is the packet-switched enhancement of the Global System for Mobile Communication GSM, which is a circuit switched network. It means that the user can be permanently online connected but it has to pay only for the real data transfer. In order to fulfil the new requirements some changes are to be introduced into the GSM. Among other new logical nodes are to be introduced, the Serving GPRS Support Node SGSN and the Gateway GPRS Support Node GGSN. The main functions of the GGSN involve interaction with external IP packet networks providing connections to Internet Service Providers ISPs via the Gi interface. The SGSN serves all GPRS subscribers that are physically located within the geographical SGSN service area. It communicates via the Gn interface, which defines the IP based backbone, with GGSN. The IP based backbone is the restriction of GPRS in that GGSN and SGSN are to be connected in a way that IP is run on top of the technology chosen, meaning that SGSN and GGSN communicate via IP addresses. This restriction applies also for the Iu-PS interface, which is defined between the SGSN and the Radio Network Controller RNC. The RNC manages Radio Access Bearers for user data, the radio network and mobility. The Radio Base Station called also Base Transceiver Station BTS or simply Base Station BS or in 3GPP, Node B, provides the radio resources and communicates with the user equipment over the Uu interface.
A detailed description of the architecture is to be found in 3GPP TS 03.60 V7.5.0 (2001-01) 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, Digital cellular Telecommunications System (Phase 2+), General Packet Radio Service (GPRS), Service Description, Stage 2 (Release 1998).
Similar nodes and interfaces are also used in the next generation of the wireless networks, Universal Mobile Communication System UMTS as described in 3GPP TS 23.060 V3.6.0 (2001-01) 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, General Packet Radio Service (GPRS), Service Description, Stage 2 (Release 1999).
In order to distinguish between the functionality of these nodes in UMTS extended names are often used, 3G-SGSN and 3G-GGSN. In the following description it will not be distinguished between the GPRS and UMTS nodes.
Currently broadcast in mobile networks causes some problems. The impacts on the problems have among other things the mobility of the end users and low transmission bandwidth of the mobile network on the air interface. Further the communication in a mobile communication networks like for example in UMTS is a unicast communication. The unicast communication is also called point-to-point communication. In such kind of network, and in particular in the core network, it is not foreseen to perform broadcast or even multicast communication. The group communication is implemented by means of a point-to-point communication having a sender transmitting separately the same packets to each receiver, instead of one packet when broadcasting is used. This means the IP broadcast messages are sent from a router settled in an external IP network, like the Internet to the mobile station via unicast connection, because from the point view of the mobile station the router in the Internet is the first node in which the IP connection terminates and therefore the first node applicable for broadcast. Therefore the broadcast is performed on the application IP layer and the network nodes between the server and the user forward merely the broadcast or multicast messages within the core part of the network without distinguishing between a broadcast message and a unicast message. The existing technology of a point-to-point oriented packet-switched telecommunication network, like UMTS does not foresee the utilisation of efficient broadcast.
Beyond this, broadcast is fine when only several users share a small LAN, but in bigger networks, like UMTS, in which a large number of users are connected by different network nodes the broadcast becomes a problem. In particular in mobile networks sending the broadcast information to all users leads to consume considerable amounts of network bandwidth. Further the user's equipment has to check the received information and in case of irrelevant information it has to discard the received packets. This causes an inefficient utilisation of the network node's resources. However there are many applications, which relay on broadcast for example broadcasting service on demand, video and music programmes, multi-camera angle sport viewing and replay or alternate scenario dramas, which are programs, which continuation depends on the result of voting of the receivers.
Often the applications using broadcast as a form of delivery depend on the geographical location of the user. Not taking geographical location into account means that the broadcast messages are sent to all users independently from the matter whether this message could be interesting for the user.
In general introducing and performing of broadcast in a protocol stack, which is basically point-to-point oriented causes problems, then in such kind of networks a unicast channel is established for performing communication between two nodes.